Calculator should always return a BigDecimal, but in same scenarios that's not happening

[stefano-sarioli]

The calculator should always return a BigDecimal, but that's not always the case.

For example the flat_rate calculator return 0 as Integer on the else branch.

If the calculator returns a type other than BigDecimal a warning will be issued by the adjustment action for the promotions,
for example create_quantity_adjustments.

I think that we should document this expected interface on the Calculator, and update our actual implementation to respect the expected interface.

I wasn't able to find a test that raises this warning on the Solidus codebase, we should probably add some tests to cover this case.

Solidus Version:
First found out on Solidus 2.5.2.

To Reproduce

• Write a test that creates a create_quantity_adjustments using a flat_rate calculator giving in input a nil object.
• Launch it.
• A Warning should appear in the console about results not being a BigDecimal. ("Spree::Calculator::FlatRate#compute returned 0, it should return a BigDecimal")

Current behavior

• Raises a warning when using a standard calculator.

Expected behavior

• Doesn't raise any warning when using a standard calculator.

[Ilyeo]

Hi! I did this PR #3789, let me know if I made a mistake or if can I improve something thanks.

[redencion]

Hi, I would like to work in this issue. Could I?
it is PR #4191

[redencion]

Hi all, I tested create_quantity_adjustments I added nil object but it is not return 0 either bigdecimal.

this is I got

So, I think as I saw in description it should get back bigdecimal, but It'll not to be, because there exist another class that catching object be nil or not, this is PartialLineItem here

If I'm lost appreciate your help, thanks